A collection of characters that group, organize, and label the pieces of content in a document. Markup tags are interspersed within the content as instructions to the parser, which removes them as it builds a document data structure in memory. Markup includes start and end tags for elements, entity references, declarations, comments, processing instructions, and marked sections. See also element, tag.
A set of formal rules for representing data and encoding structures that surround the data. A document that obeys the rules of the markup language is said to be well-formed. A markup language provides ways to label parts using elements, to enforce structure using a DTD, and to import data with entity references. XML is not itself a markup language, but a set of rules for creating markup languages. See also application, DTD, markup.
Descriptive data that is not directly included in the content of a document. For example, a document's creation date or author is metadata. See also document, markup.
A mixture of elements and character data that can be specified as valid content for an element via its content model. See also CDATA, content, content model, element.
A DTD that is divided into logical pieces, allowing for easy maintenance and selection of only the modules required in a document. The modules of a modular DTD are often kept in external files and declared as external entities. See also DTD.